The requirements for energy savings and safety of lubricant oils have become increasingly stringent in recent years. In terms of energy savings, base oils are required to have high viscosity indexes in order to lower the viscosity to a practical range, while maintaining viscosity at the maximum design temperature of the device. In industrial fields, in particular, ISO VG32 grade oils with a kinematic viscosity at 40° C. of approximately 32 mm2/s are primarily used, and properties in this viscosity range are desired. In recent years, base oils with lower low-temperature viscosities have been desired for the purpose of lowering the viscosity resistance at device cold-start. Improvement in the low-temperature characteristic is currently achieved, in most cases, by adding a pour point depressant or the like to a lubricating base oil such as a highly refined mineral oil, to improve the low-temperature characteristic (see Patent documents 1-3, for example).
From the viewpoint of safety, on the other hand, it is desirable to remove heavy metals in the lubricant oil, while also lowering sulfur content. In addition, the Japan Fire Service Law places strict limits at factories and the like, regarding total factory holding volumes at the flash points of lubricant oils, and therefore in consideration of handling issues there is a strong demand for lubricant oils with flash points of 250° C. and higher which are non-hazardous substances according to the Japan Fire Service Law.
Known processes for producing high-viscosity-index base oils to meet this demand include processes in which feedstock oils containing natural or synthetic normal paraffins are subjected to lubricating base oil refining by hydrocracking/hydroisomerization (see Patent documents 4-6, for example). There are also known methods for producing lubricating base oils designed with high flash points and lubricating oil compositions with high flash points, which avoid sacrificing other properties (see Patent documents 7-8, for example).
The properties evaluated for the low-temperature viscosity characteristic of lubricating base oils and lubricant oils are generally the pour point, clouding point and freezing point. Methods are also known for evaluating the low-temperature viscosity characteristic for lubricating base oils, according to their normal paraffin or isoparaffin contents.    [Patent document 1] Japanese Unexamined Patent Application Publication HEI No. 4-36391    [Patent document 2] Japanese Unexamined Patent Application Publication HEI No. 4-68082    [Patent document 3] Japanese Unexamined Patent Application Publication HEI No. 4-120193    [Patent document 4] Japanese Unexamined Patent Application Publication No. 2005-154760    [Patent document 5] Japanese Patent Public Inspection No. 2006-502298    [Patent document 6] Japanese Patent Public Inspection No. 2006-502303    [Patent document 7] Japanese Unexamined Patent Application Publication No. 2004-250504    [Patent document 8] Japanese Unexamined Patent Application Publication No. 2004-182931